下面的getInstance（）方法有什么问题 [英] What's wrong with the following getInstance() method

问题描述

我一直在学习求职面试，我发现这个问题：

I've been studying for a job interview and I've found this question:

下面的Singleton工厂方法getInstance（）有什么问题？

What's wrong with the following Singleton factory method getInstance()?

public class Singleton {

    private static Singleton mySingleton;

    protected Singleton() {}

    public static Singleton getInstance() {
        if (mySingleton == null) {
           synchronized(Singleton.class) {
             if(mySingleton == null) {
                mySingleton = new Singleton();
             }
           }
        }
        return mySingleton
    }   
}

我知道构造函数是错误的，应该是私有的。但是，他们问的是getInstance（）方法有什么问题，一切对我看起来不错，我看过很多这样的例子（在多线程环境中）我缺少什么？

I know the constructor is wrong and should be private. However, they're asking about what's wrong with the getInstance() method, everything looks fine to me, I've seen a lot of examples like this one (in a multithreading environement) what am I missing?

推荐答案

原始代码最大的问题是它是反复检查反模式的典型示例。它看起来不错，但只是坏了。更多相关信息：
http：//www.cs .umd.edu /〜pugh / java / memoryModel / DoubleCheckedLocking.html

The largest problem with the original code is that it is a classic example of the Double Check anti-pattern. It looks great but is just broken. More on that here: http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html

请注意，从Java 1.5开始，模式或更好的静态初始化真的是走的路。

Note that as of java 1.5 there is a way to finesse it, the synchronized pattern or better static initialization is really the way to go.

-edit
（我只是注意到这是一个求职面试，如果他们显示这个例子，对于细节，我会去与复杂的java内存问题，你没有完全理解，但接受事实，以避免双重检查。

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